278 FUNCTIONS OF NUTRITION. 



the admixture of another element in its production, namely, the muscu- 

 lar contraction of the ventricular walls. But from the evidence thus 

 far presented, it appears that the direct share of muscular contraction 

 in the first sound, if it exist at all, must be secondary in importance to 

 that of valvular tension. 



The cause of the second sound is universally acknowledged to be the 

 sudden closure and tension of the aortic and pulmonary valves, under 

 the reaction of arterial pressure at the end of the ventricular systole. 

 These valves are fibrous, sernilunar festoons, which yield to the current 

 of blood passing from the ventricle into the artery (Fig. 59), and which 

 shut together in the form of distended sac& (Fig. 58) when the artery 

 reacts upon its contents. Their connection with the second sound 

 of the heart, which occurs at the same time, is established by the 

 following proofs : 1st. The sound is heard with complete distinct- 

 ness directly over the situation of these valves at the base of the 

 heart ; 2d. The farther we recede in any direction from this point, 

 the fainter it becomes; and 3d. All experimenters agree that when 

 the semilunar valves are hooked back against the inner surface of the 

 artery by curved needles, or held open by fine springs introduced into 

 the vessel, the second sound disappears, and remains absent until the 

 valves are again liberated. 



The difference in character between the first and second sounds of the 

 heart is apparently due to the difference in size and attachment of the 

 auriculo-ventricular and the semilunar valves. The former are com- 

 paratively broad sheets attached by their external edges to the auriculo- 

 ventricular fibrous zones, and by their internal edges and lower sur- 

 faces, through the chordee tendinese, to the musculi papillares of the 

 ventricular walls. The latter are of smaller size, and attached only to 

 the fibrous zones at the base of the large arteries. In imitating the 

 effect of valvular tension with a piece of ribbon or other woven fabric, 

 a longer piece will yield a sound similar to the first sound of the heart, 

 a shorter piece one similar to the second sound. 



The movements of the heart may be observed in the dog, or other 

 warm-blooded quadruped, after opening the chest by a longitudinal 

 incision through the sternum and separating the costal cartilages on 

 each side at their junction with the ribs ; artificial respiration being 

 maintained by the nozzle of a bellows inserted into the trachea. The 

 animal may be etherized and rendered permanently insensible by tre- 

 phining the skull, and applying cerebral compression, or he may be par- 

 tially narcotized by a preliminary subcutaneous injection of morphine, 

 after which etherization is produced, and continued with great facility. 

 The operation of opening the chest- and exposing the thoracic organs 

 increases the rapidity of the heart's movements, and diminishes their 

 force ; but they often have sufficient vigor to continue with regularity 

 for one or two hours, if artificial respiration be properly maintained. 



When exposed to view by this means, the action of the heart is so 

 complicated that it requires a close examination to appreciate its char- 



